In the many years of the use of radiation sources for irradiating objects or (in the medical field) patients, the design of radiation equipment has taken into account the safeguards for personnel in placing and using radiation sources. Usually this involves the presence of large bodies of lead surrounding the radiation source with provision for limited exposure of the radiation source to provide the required radiation followed by shielding at the end of the radiation period.
Often the shielding takes the form of a full shielded room. More often it involves a shielded room plus shielding in the equipment itself. Much of such equipment is configured such that it must be shipped without a radiation source in place and is then installed. The radiation source is then inserted within a shielded room or within temporary lead barriers with remotely controlled manipulatory means.
Many such pieces of radiation equipment have been installed throughout the world, used and the radiation sources have become exhausted from the operational standpoint but are still hazardous to unshielded personnel. An example of such an application involves systems for irradiating a patient's cranium with gamma particles from a Cobalt 60 source. Such equipment has been known and widely sold by Eleckta Corporation of Sweden and is known as Gamma Knife apparatus. Cobalt 60 has a half life of 5.2 years and a practical period for removal and replacement of the radiation source has been determined to be 10 years. In one such type of equipment, approximately 200 radiation sources are located within a hemispheric shield. Typically this requires the steps of:
1. setting up a portable shielded facility with a viewing window and remote handlers in the room in which the equipment is located;
2. raising the top cover of the apparatus;
3. removal of screw fasteners and restraining pins which secure aluminum source holders within a steel primary shield;
4. grasping and removing the aluminum source holder with tongs using the remote handlers or unthreading the aluminum source holder from the primary shield; and
5. transferring the source in the aluminum source holder with remote tongs to a shielded shipping container.
Steps 3-5 must be repeated for every source of which there may be 179 or 201 in currently available equipment.
Because of the tight tolerances employed in manufacturing the Cobalt 60 source, the aluminum holder in which the source is mounted and the steel or cast iron primary shield into which the source mounted in the aluminum holder is placed, which is typically 0.1 mm for the complete assembly, and because of the corrosion which occurs over a five to ten year period of use before source reloading, it has been found that sources often cannot be removed using the above standard techniques. Typically, a significant portion such as 50 percent are not removable using the standard technique. Thus there is a need for a method and apparatus capable of removing these sources.